Recovery of solubles from flue-dust



R.' B. STRINGHELD.

RECOVERY OF SOLUBLES FROM FLUE DUST.

APPLICATION FILED DEC. 7. 1917.

UNITED STATES PATENT OFFICE.

RAYMOND B. SI.I.'-KCINGFIELI),P 0F LOS ANGELES, CALIFORNIA, ASSIGNOR TO INTERNA- TIONAL l:Ell'tll'llCIPI'IA'J!ION' COMPANY, OF LOS ANGELES, CALIFORNIA, A CORPORA- TION 0F CALIFORNIA.

RECOVERY OF'SOLUBLES FROM ELITE-DUST.

Specication of Letters Patent.

Patented June 29, 1920.

Application filed December 7, 1917. Serial No. 206,122.

To all whom t may concern.'

Be it known that I, RAYMOND B. STRING- FIELD, a citizen of the United States, resid- ,I

ing at Los Angeles, in the county of Los -Angel'es and State of California, have invented a new and useful Improvement in Recovery of Solublesfrom Flue-Dust, of

which the following is a specification.

This invention relates particularly ,to the recovery of potassium compound from fiuel dust-for example, from dust or fumes coming from cement kilnsand containing potassium compound in appreciable uantity. Such dust orl fume is now` success lly re-I covered, for example, by electrical precipis tation, and the recovery of the potassium compound content thereof is of considerable y to produce from the dust, a solution of potassium salt or saltsand an insoluble or substantially insoluble residue containing. little or no potassium compound. In the vcase of cement kiln fluedust, there is always considerable calcium compound present together with the potassium compound, and in general, a considerable proportion of the potassium compound present is in the form of sulfate.

In the ordinary process of extracting potassium compound from flue dust containing potassium sulfate and calcium compound, by leaching the dust with either cold or warm water and, after filtration, and

concentrating the resulting solution, it is found that a portion of the potassium salts present may remain in the leached residue or filter press cakes, evenafter washing, which phenomena is attributedv to the formation of a relatively insoluble double sulfate having the formula CaSO,.K,SO,.H2O, causing consequent loss of potassium compound and low extraction.` V

It has been attempted to overcome this diliiculty by using, in 'leaching the fiue dust, water at-atemperature approaching 100 C., such temper'aturebeing maintained during Aboth leaching and filtration, in order to prevent the formation 'of this relativel insoluble double salt.- lSuch a recess is, however, open tothe objection, t at the production and maintenance of such high temperature,

involves considerable expense; is destructive on the apparatus, particularly the filtration devices; and interferes with the production of an ei'ective suction in the filter.

The main object of the` present invention is to provide for successful leaching or extraction of the potassium compound from such fiue dust in'such manner as to avoid any loss .due to formatlon of such double salt of potassium and calcium, and at the same time, to provide for the use of moder-- ate temperatures in extraction and filtration, thereby rendering the process more practicable and economical than where relatively high temperatures areused.

Another object of the present invention is to provide for extraction and separation of potassium compound in the most rapid and economical manner b providing for production of a substantialiy saturated solution of potassiumcompound in the leaching operation, and at the same time, avoiding the dif- I culties ordinarily arising from the production of such saturated solution.

My process may be carried out 1n any sulty'able form of apparatus, for example, in

forms shown in accompanying drawings,

wherein-- v Figure l is a iiow sheet of one form of such apparatus embodying filtration as one 0f the steps in extractiom Fig. 2 is la iiowl sheet of an apparatus adapted to dispense'with filtration andl emplolying only counter-current washing.

, he apparatus shown inl F ig. 1 com rises a' leaching means 1, preferably constltuted by an electrical precipitator of the type wherein the fume -or dust is preci itated,

by electrical action, into streams o water or dilute solution running down on vertically arranged collecting electrodes. Any other suitable apparatus, however, may be used for bringing the fume or dust into contact with water or dilute solution for dissolving the solubles therein. In general, the cement kiln dust or other dust or fumes,

which it is desired to treat, containspo-ltassium compound, partly or wholly in form of sulfate, together with calcium compound,

and I will describe my invention as particularly ,applied in connection with such fumes or dust. In' thesolution of the solubles in such fumes or dust, it is desirable during to apply more or less heat to the material of an electrical precipitator used as a combined leaching and precipitating apparatus,

the requisite heat Will, in general, be furnished by the kiln gases which carry the fume or dust, and which enter the apparatus 1 at' inlet indicated at 2 and leave the same at `outlet indicated at 3, said gases being generally at high temperature. apparatus .1,A all of the collected' material, such as fume or dust, together with the v leaching liquid supplied to the apparatus sum v 19.y The leaching is so carried out- 41n t e apparatus 1 that a substantially saturated solution'of potassium sulfate-say,

. guivalent to about 4 per cent. calculated 4as O--is produced-and the decanted liquid is of this same concentration so that itis in the most available condition for. economical vrecovery of the potassium compound, either bycrystallization or by evaporation to dryness.

I prefer to carry .on the leaching operation and the thickening operation at a temperature not in excess'of 80 C. and prefery ably in the neighborhood of 50 C., as such to suitable liltermeans 11-'preferablya coni l vso a temperature is most easily maintained and does not .requirespecial application of heat, or special means for retaining the heat in the solution. At Such temperatures, and

with the concentration of potassium sulfate -statednthere 1s; in, general, a tendency to.A formation of the -insoluble double salt above referred to, said salt collecting along with 'the other insoluble material lor sludge at the Abottom ofthe thickener and being drawn o' with this sludge through outlet connec-r tion 7, to suitable receivingmea'ns such as agitator 8... Water or dilute solution of po'- ta'sslum compound is added to this agitator through-a suitable connection indicated at 9.' Thev outlet 'pipe 10 ofthe agitator 8 leads tinuous filter of anyof the well known types, adapted for filtration vand washing under suction.y -The linsolubles are removed continuously from the filter in the usual manner and conveyed .away'at 12 by suitable conmeans, andthe wash water'which is supplledy at 13, asses with theremainder ofthe solution rom the .filter through .an

outlet connection 14 to a suitable receivlng the leaching operation. In the case From the means, or sump 19, being preferably con- -ducte'd back from said sump by suitable pump means 15 and piping 21, to inlet means l16, for supplying liquid to the leaching vapparatus 1.

' In the general art and practice of potassium compound recovery by leaching cement plant iue dust, it has-been the practice in this, as well asin other lixiviation processes, to bring the extracted Solution to the highest possible concentration of the desired salt during the lixiviation process, in order to permit either theseparation by fractional crystallization on cooling, or to reduce the cost of evaporation Where the ventire dissolved Salt is recovered by the evaporation process. '1

It has been found that in leaching cement plant iue dust containing substantially no chlorids, efficient extraction of the KZSU4 content ceases when the solution has reached a potassium compound concentration I.slightly in excess of 4% calculated as KO,

and atA this concentration and temperature, under 85, the double salt of CaSOK2SO H2O may form .and separate out in solid form, with considerable rapidity and in con-l siderable amount, interfering with the economic recovery o f the potassium compound contained in the dust.

I have found that the presence of large quantities fof CaSO4,K,bO4,H2O crystalsl in a pulp, because of their special crystal form (needle-like and in clusters of pure salt, much resembling lass lWool in aspect) create most unfavorab e conditions for filtration. The CaSO4,K2SO4,I-IZO crystals apparentlyti'e the solids together to form a mass, which handles on the filter almost like a colloidal' precipitate, rendering difficult the proper separation of solidand solution by filtration, and adding greatly to the difiiculties of proper Washing.

Investi ation has shown that the formation of this insoluble double salt (MS04, KZSO4H2O` may be completell at temperatures below 859A by o prevented Werlng the potassium compound concentration so as' to fall to or belowapproximately the followving temperature concentration ratios (the potassium beingv calculated Vas KZO) :4

15,1% K,o a 25fe. Vcairo at 32C. 3.1% Klo at eef yC.

' My resent process is based .the utiliaaytion o this fact that the formation and separation ofthis insoluble double salt' may be prevented by suitably reducing the. concentration of potassium salt at a certain 'stage inthe extraction, this being effected in such manner as not to reduce the concentration'. of the solution utilized yfor production by evaporation 'or crystallization of the potasysium compound. For this purpose, I carry 130` ferred to, which may have been formed,4

with the result that the potassium compound is restored to the solution, which may then be removed from the insoluble material'by .filtration or otherwise, and the potassium compound obtained therefrominany suitable manner. In practice, I prefer to return the dilute solution so'obtained, to the leaching stage of v,the process, for cyclic operation.

My process is preferably carried out as follows:

The dust containing potassium com pounds is leached in apparatus 1, at mod-I erate and economical temperature rangesay, between 50 and 75 C., thus securing a solution'containing -substar'itially 4% potassium compound calculated as KZO, passing all the4 solution with insolubles carried thereby, to the thickener 5, wherein the insolubles settle out; decanting the supernatant sulfate solution of substantially 4% strength calculated as K2() solution, and passing said solution in part or in' whole, through outlet 6 to evaporators or crystallizing tanks or pans. consists of from 1 to 2r parts of solution of approximately 4% K2() content, .to 1 of solids by weight; and may hold a considerable amount (dependenty in part, upon length of contact of solvent and flue dust, and in part, uponpotassium compound concentratlon), of'potassium compoundV as the double salt CaSVO4,K2SO4,H2(). This sludge is then passed to an agitating tank into which fresh water or a more dilute potassium compound bearing solution is added in such amount, and either hot or cold, that the. total KZO concentration of the agitator pulp is 3%, or preferably under, if the temperature is 80; 2%, or preferably under, if the temperature is approximately 30, and in a proportional ratio for any tem erature between these two, or below 30 with the immediate result that thedouble salt (NSOAT. ,SO4,H2O is decomposed. The decomposition, with decreasing KZO 4concentration in the solution, proceeds along either one of two routes, depending upon the prevailing temperature. Above 30 to 35 C., and concentration between 2 and 3% K2O, the break-down of `the double salt first results in the formation of another double salt (potassium pentacalcium The settled sludge' sulfate) containing only one-fifth as much KZSO4 as the primary double salt, and of an entirely different physical character and less objectionable in the filtering operatlon than the mono-calcium double salt.v On further dilution, such as would be encountered in washing theY filter cake, or in Washing the solids by the principle of countercurrent flow, the potassium pentacalcium sulfate becomes unstable when the concentration, at 'sli htly over 30, is approximately 2% an atf70, approximately 1}% KZO. .This decomposition of the potassium .pentacalcium sulfate results inthe precipitating out of gypsum or of anhydrite salt, while the entire 'potassium content is returned. to the solution as a simple soluble compound K2SO4. Below approximatel 30 C., the decomposition' of the double sa t results directly in formation of insoluble gypsum and soluble K2SO4, the CaSO,.2H2O

l becoming a constituent of the solids in the sludge and the K. ,S()4L returning to the sointion in dissolved condition. rlhe breaking 111g) of the double salt and re-solution of the ZSO4 by either of these routes requires an appreciable time, as the operation is a surface reaction, andthe agitation ofthe sludge with additio'nalwater is continued for suffi. cient time to insure completion of such resolution.l The' sludge is then passed to filter 11, 4wherein the insolubles are removed at '12 and the solution at 14. Said solution, being dilute, is preferably returned to the leaching apparatus 1 by pump 15.

My invention is especially a plicable Iin connection with the process ci) potassium compound recovery from cement kiln fille dust, disclosed .in the application of HarryA V. Welch, filed August 11,1917, No. 185,779, such process'being carried out in an elec' trical precipitator, the collecting electrode of which is a continuously moving. sheet or film of water, or of solution supplied by' suitable water supply lmeans atthev treater ltop, as indicated at 16 in Fig. 1, this moving electrode sheet or film being maintained and the potassium compound contenty controlled within certain limits by utilizing a closed moving liquid circuit'from the discharge at bottom of the collecting electrodes to thetop and feed of the collecting electrodes.- Forthe separation of essentially the inert solid constituents of the collected flue dust, the -thickener device 5 (ora filter means), .is introduced in this closed circuit, the main part of the overflow from this thickener to sump 19 being returned by Vpiping 21 and pump 15, to the liquidv supply means at the top of the precipitator. The operation is such as to remove at each cycle, after the ldesired potassium compound concentration .had been reached, a certain fraction of the solution through the pipe 6, and remove substantially all the solids through outlet 7, while returnto two to four of solvent, is practised, de-v pendent, in particular, upon the available water soluble potassium compound in the fiue dust and the limit of approximately 4% potassium compound in the solution calculated as KzO, and' in particular, upon the necessity of mechanical handling. In the case of leaching by precipitation with {iowing liquid as described, the ratio of dust to liquids may reach 1 of solids to 50, and up to 100 of liquids. A portion then of the clear overfiow from the thickener device is sent through outlet 6 to evaporators or to crystal.-

lizing tanks or`pans, indicated at' 17, while4 the thickenedpulpdischarge from the thickener device and running approximately l of solids to 1 to 2 of liquid, is passed -to` the agitator tank 8, where the dilution is brought up to 4 or 5 of liquid to 1 of solids, and the potassium compound concentration calculated as K2() lowered to 3% or lower for 80, and 2% lor lower for 30, underwhich conditions, the double salt CaSO4,K2SO4, H2O, is completelydecompos'ed, and the solution of potassium compound is then filtered from the insoluble material, as above described, and returned to the precipitator and leaching `apparatus 1, through piping 2 bypump 15.

y invention may also be carried out in the apparatus 'shown in Fig. 2, wherein the leaching means 1, `preferablyan electrical precipitator such as above described, discharges through outlet 4 to a decantation or dewatering tank 5, preferably formed as a thickener, 'theoverfiow from this thickener passing by connection 6 to evaporator or crystalllzer and by connection 23 to a sump `25, from which it is pumped through pipe 27 by pump 2.6 back to the supply means 21 for the leaching apparatus. The sludge from the thickener 5 is passed by suitable conveying means 28 to the iirst of a series of thickeners or washing and dilution tanks 29, 30, 31 and 32, each of which has its sludge outlet 33 connected to discharge into the following thickener, and each of which has its overiow outlet. 34 for cleary liquid, connected to discharge` intothe sludge intake of the preceding thickener so as to Wash the sludge by counter-current action, the' wash water being supplied to the last thickener of y the series by connection 35 and the overow Leaaoea ener 29 where it is diluted sufficiently by dilute solution coming through inlet connection 34 to reduce the concentration of potassium sulfate in the solution to a point suffciently low to break up double sulfate of potassium and calciumthat may have been formed and bring substantially all of the potassium sulfate into solution. The efl'ect of the series of succeeding thickeners 30, etc., is to wash the remaining insolubles thoroughly and carry all of the solubles into and through the first thickener 29, and out at the overflow 36, which delivers the same to the sump 25 from which the leaching apparatus is supplied. Thickeners 29, etc.,"w hich in this case serve as mixing and washing means, may be of any suitable type, such as Dorr thickeners, and may be of any desired number, referably at least three, so as to provide or effective counter-current washing. The breaking up of the insoluble double salt is effectedmainly in the lfirst thickener or dilution tank 29, but the following thickeners 30, 31, etc., may also serve to break up further quantities of the double salt, and to dissolve the contained potassium sulfate'. For example, the dilution in tank 29 may serve for breaking up OaSOK2SO, H2O forming potassium sulfate, which dissolves, and the pentasulfate .which .is relatively insoluble, but is decomposed in further dilution in the succeeding tanks 30, etc.

y Moreover, since the breaking `up of the double the insoluble material and adding to saidv insoluble material enough water to reduce the potassium sulfate concentratlon of the mixture sufficiently to decompose said it may not be completed until the mav has had time to travel through sevdouble salt and bring a further quantity of potassium sulfate into solution, and treatmg the resulting dilute solution, to recover the potassium compound therefrom.

2. The process of recovering potassium compound from solid material containing potassium, calcium and the sulfate radical, which consists in treating such material with water, in such manner as to produce a solution containing potassium sulfate and calcium sulfate in such concentration as to also produce a relatively insoluble double sulfate of potassiumand calcium, then separating said concentrated solution from the insoluble material and adding to saidginsoluble material enough water to reduce the potassium sulfate concentration of the mixture sufiicientlyto decompose said double salt and bring a further quantity of potassium sulfate into solution and utilizing the resulting dilute solution in treating a further quant-ity of the solid material.

3. The process'of recovering potassium compound from cement kiln dust containing potassium sulfate, which consists in subjecting such dust to the action of Water at a temperature below 8Oo C., and in such manner as to produce a solution containing potassium sulfate and calcium sulfate in such concentration as to alsoresult in the production of an insoluble double sulfate of potassium and calcium, recovering such concentrated solution from the sludge containing the insoluble material, treating said solution to obtain potassium sul-fate, diluting said sludge suiiciently to decompose the double sulfate of potassium and calcium contained Vtherein and to produce a dilute solution of potassium sulfate, and utilizing such dilute solution in treatment of a further quantity of dust. A 4. T he process of separating potassium compound from a mixture of solution and insoluble material at a temperature below 80 C., and containing potassium sulfate and calcium sulfate in such concentration as to cause the formation of a relatively insoluble double salt containing sulfates of potassium and calcium, which consists in first removing a portion of the solution from the mixture, and diluting the residual mixture suffiying such concentrated solution from the insoluble material, and subjecting the remaining solid material to the action of Water under such conditions of dilution of potassium salt, and for a suiiicient time, to decompose such double sulfate and bring a further quantity of potassium sulfate into solution.

6. A process of leaching flue dust from cement manufacture, containing potassium compounds, a part of which are in a relatively .insoluble condition, which comprises subjecting such flue dust to systemat1c extraction in a series of successively more and more dilute aqueous solutions, at least some of which are hot, and maintaining contact of the said flue dust and aqueous solvent until hydrolysis of a considerable part of the relatively insoluble potassium compounds is eifected, whereby the relatively insoluble potassium compounds are, in part at least, rendered soluble and dissolved in the said solvent.

In testimony whereof I have hereunto subscribed myname this 23rd day of November, 1917.

RAYMOND B. STRINGFIELD. 

